The present invention relates broadly to a data transmission system, and in particular to a synchronous time division digital multiplexer apparatus for use with a modem transmission system.
In the prior art, the transmission of digital data over telephone lines by frequency modulations-demodulation techniques is well known. Frequency modulation techniques in which a carrier wave is shifted between two frequencies during successive modulation periods are commonly called frequency shift keying frequency shift keying systems. In these prior art systems, the carrier wave is generated at the transmitter by means of well known analog techniques which utilizes resistors, inductors, etc. Analog carrier wave generators of such prior art transmitters are unstable to some degree which causes unwanted shifts in the transmitted carrier frequency. The stability of such analog circuits may be improved to some extent by the use of crystal controlled oscillators and temperature compensating devices. However, such devices are not only expensive, but do not completely eliminate the instability problem. Furthermore, the close harmonics of the carrier frequencies, for example, third and fifth, are difficult to remove and may result in errors in the demodulated signal.